(a) Field of the Invention
The present invention relates to an apparatus and a method for compensating carrier feedthrough in a quadrature modulator and a quadrature modulation system.
(b) Description of the Related Art
In general, an ideal voltage difference in a quadrature modulator is considered to be when the DC voltage difference of a baseband differential input is 0, since this prevents carrier feedthrough. However, the DC voltage difference of the baseband differential input in the quadrature modulator is not substantially 0, and this causes carrier feedthrough due to mixing with a local oscillator signal.
Currently, carrier feedthrough occurs even in a quadrature modulator used in a system using quadrature modulation, the carrier feedthrough being caused by the DC voltage difference of a baseband differential input. The carrier feedthrough occurs differently as various types of the quadrature modulators. For example, there are quadrature modulators of which the carrier feedthrough is at the minimum when the DC voltage difference is 0, whereas there are quadrature modulators of which the carrier feedthrough is at the minimum when a slight DC voltage difference appears.
A digital compensating circuit has been used in the related art to suppress the carrier feedthrough. In more detail, it detects a carrier feedthrough first, digital circuit compensating carrier feedthrough is activated by comparator and the carrier feedthrough is compensated by a DC compensation signal through a digital-to-analog converter. However, according to the above method, because a number of digital blocks, i.e., constituent elements including a comparator, a digital-analog converter, a control signal generator, etc., in addition to the quadrature modulator are needed, the configuration of the system becomes complicated.
Another method has been disclosed in the related art that detects a DC voltage error in an in-phase/quadrature-phase input in a modulator, and then compensates carrier feedthrough using a bias current of a transistor that performs a modulating operation. However, this method can be applied to only the design of a quadrature modulator.
Another method has been disclosed in the related art that suppresses carrier feedthrough at in-phase/quadrature-phase using a correlator, an integrator, and a pseudo-noise generator on a feedback path. However, the configuration is too complicated to implement a device that suppresses carrier according to this method.